1. Field of the Invention
The present invention relates to a pharmaceutical agent which inhibits replication of virus by increasing a specific enzymatic activity of liver and/or other tissues.
2. Description of the Related Art
Virus has glycoproteins and glycolipids in viral membrane. Glycolipids are derived from cell membrane of host cells while the protein part of glycoproteins is a virus-specific gene product. Sugar chains are added to the protein part in the Golgi apparatus of host cells and, in this process, a viral enzymatic system is not used but an enzymatic system of the host cells is used. For example, there is acquired immunodeficiency syndrome (AIDS) which is a disease where a kind of retrovirus called human immunodeficiency virus (HIV) infects mainly on cells in which molecules of CD4 are expressed whereby CD4-positive T cells are significantly decreased and, in infection of HIV, molecules of gp120 which is a virus envelope glycoprotein play an important role. About one half by weight of the gp120 molecules is occupied by sugar chains and there have been various reports on the importance of those sugar chains during the course of infection of HIV. For example, it is reported that the gp120 molecule lacking the sugar chain loses an ability of binding with CD4. It is also reported that syncytia are formed when HIV-infected cells are cultured together with uninfected cells and that addition of molecules of gp120 inhibit said syncytium formation while addition of molecules of gP120 lacking sugar chains does not cause an inhibition. It is further reported that both E1 and E2 which are thought to constitute the envelopes of human hepatitis C virus (HCV) are glycoproteins and both of them have no sialic acid residue at the end of the sugar chain but a few of them have N-acetylglucosamine residue at the end. Accordingly there is a possibility that liver is infected with HCV via asialoglycoprotein receptor on hepatic parenchymal cells or via mannose-binding protein found in hepatic endodermic cells or macrophage. Furthermore, human hepatitis B virus (HBV) has glycoprotein called an HBs antigen on the surface of its particle where said protein has two N-glycans linked type and it is reported that those sugar chains play an important role in each of the steps of viral replication, translocation and secretion.
Up to now, there have been shown several possibilities of antiviral agents where attention is paid on the sugar chains of those viral glycoprotein. For example, it is reported that syncytium forming ability and virus infecting ability disappear in HIV-infected cells which are cultured in the presence of an inhibitor for N-glycan processing enzyme such as tunicamycin. For example, there is a report on the inhibition of secretion of human HBV by N-butyldeoxynojirimycin which is an iminosugar [Proceedings of the National Academy of Sciences of the U.S.A., 91, 2235-2239 (1994)].
However, a treatment by such an N-glycan processing inhibitor disturbs glycosylation of the host cells and, therefore, the sugar chain structure of glycoprotein of the host cells is naturally affected to a great extent and that is never satisfactory in view of safety.
During the course of studies on the structural change of sugar chain on the cell surface, the present inventors succeeded in obtaining N-acetylglucosaminyltransferase III (GnT-III) of rat and human (Japanese Laid-Open Patent Publications Hei-6/38,767 and European Patent No.585083). This enzyme produces the so-called bisecting GlcNAc, i.e. a GlcNAc.beta. 1-4Man.beta. 1 structure of N-glycan.
The present inventors have found already that, in an experimental system using LEC rats which give a natural onset of hepatitis and hepatic cancer, activity of N-acetylglucosaminyl-transferase V (GnT-V) and that of GnT-III significantly increase as compared with the use of LFA (a control rat) in the third stage which is a stage of onset of hepatitis and in the fourth stage where cancer tissues are macroscopically observed in liver, respectively. It has been also found that this GnT-III rarely appears in normal liver but, in a chemical carcinogenic process in rat, the activity increases in cancerous site, precancerous change site, cells derived from ascitic cancer, fetal liver, regenerated liver, etc. It has also been disclosed by the present inventors that an expression of enzymes in a serum of a patient who is suffered from hepatitis, hepatocirrhosis, tissue of hepatic cancer, or hepatic diseases thereof is increased. [Biochemical and Biophysical Research Communications, 152, 107-112 (1988); Clinica Chimica Acta, 185, 325-332 (1989)].
As mentioned already, it has been known that the activity of glycosyltransferase changes in virus-infected cells. However, no method in which such a phenomenon is utilized as a site of action whereby replication of virus is inhibited and viral diseases are treated has been developed yet.
As a result, object of the present invention is to offer a pharmaceutical agent in which a specific enzymatic activity in liver and/or other tissue is increased so that replication of virus is inhibited.